The present invention relates to a device for detecting tire air pressure based on a vehicle speed signal.
JP-A-H7-125512 discloses a conventional tire air pressure detection device. The tire air pressure device can detect an accurate tire air pressure by removing the effect of tread-lift. Tread-lift is a phenomenon wherein a dynamic rolling radius of a tire in which tire air pressure has decreased becomes approximately the same as that of a tire in which tire air pressure has not decreased because the radius of tire increases due to a centrifugal force.
When the dynamic rolling radius of the tire increases due to the effect of tread-lift, a determination value for determining tire air pressure decrease is erroneously determined because the dynamic rolling radius of the tire in which tire air pressure has decreased becomes approximately the same as that of a tire of which tire air pressure has not decreased. That is, the determination value for determining tire air pressure decrease varies to close that of a determination value of the tire of which tire air pressure has not decreased. As a result, tire air pressure is not detected accurately.
Accordingly, in JP-A-H7-125512, an air pressure detection device checks a relationship between vehicle speed correlating with the centrifugal force and a determination value for determining the tire air pressure (e.g., a rotational status value), and stores the relationship in memory. Thus, the determination value is compensated for with respect to vehicle speed based on the stored relationship, and therefore tire air pressure is detected accurately.
However, a degree of the effect of the tread-lift also varies with respect to the tire air pressure. For example, a relationship between the tire air pressure and the effect of the tread-lift is shown in FIG. 22. As tire air pressure decreases, the dynamic rolling radius due to the effect of tread-lift also increases. Incidentally, the dynamic rolling radius xcex94R is expressed in, for example, xcex94R=10(xe2x88x921.97xe2x88x921.31)xc3x97V(0.735pxe2x88x921.147), where tire air pressure is P(kfg/cm2) and a vehicle speed is V(km/h).
Therefore, although the effect of tread-lift is not generated when tire air pressures of four wheels are identical, it is generated when one of the tire air pressures of the four wheels significantly decreases. As a result, if the determination value is compensated for as mentioned above when the tire air pressures of four wheels are identical, non-uniformity of the determination value increases.
JP-A-H10-100624 also discloses a conventional tire air pressure detection device. The tire air pressure device detects a decrease in tire air pressure based on wheel speed variation D and a front and rear wheel speed ratio xcex2. The wheel speed variation D and the front and rear wheel speed ratio xcex2 are expressed as follows, where VFR corresponds to front right wheel speed, VFL corresponds to front left wheel speed, VRR corresponds to rear right wheel speed and VRL corresponds to rear left wheel speed.                     D        =                                            V              FR                                      V              FL                                -                                    V              RR                                      V              RL                                                          (        1        )                                β        =                                            V              FR                        +                          V              FL                                                          V              RR                        +                          V              RL                                                          (        2        )            
The wheel speed variation D represents a rotational status value calculated based on wheel speeds of four vehicle wheels. For example, the wheel speed variation D is a variable defined as a difference of wheel speed ratios of each pair of wheels located diagonally from each other, and increases or decreases when the tire air pressures of some of the vehicle wheels decrease. The front and rear wheel speed ratio xcex2 is a tire slip status value that denotes a degree of slip status of driven wheels caused by transmitted driving forces. For example, the smaller the front and rear wheel speed ratio xcex2 is, the higher the slip of one (or both) of the driven wheels is.
The wheel speed variation D increases or decreases when the tire air pressures of some of the vehicle wheels decreases below a standard value, and it is zero when each tire air pressure of each tire equals the standard value. Therefore, the tire pressure decrease is detected based on the wheel speed variation D.
However, regarding, for example, a rear wheel drive vehicle, when the tire air pressure of the rear right wheel corresponding to one of the driven wheels decreases below the standard value, the other driven wheel tends to slip easier than the rear right wheel because a ground contact area of the rear right wheel increases and resistance force for suppressing the slip increases even if the diameter of the rear right wheel decreases due to the tire air pressure decrease. Accordingly, the wheel speed variation D varies based on the degree of slip status of the wheels.
Thus, as shown in FIG. 23, a regression line is calculated based on a relationship of the front and rear wheel speed ratio xcex2 and the wheel speed variation D using a minimum square calculation methodology. An ideal value of the wheel speed variation D is then calculated by compensating for the wheel speed variation D (or an average DAVE). An ideal value of the wheel speed variation D is a value of the wheel speed variation D if the slip does not occur when the front and rear wheel speed ratio xcex2 is 1. Thus, the effect of the slip of the driven wheels is removed, and therefore the tire air pressure decrease can accurately be detected.
However, the wheel speed variation D varies based on the effect of the tread-lift as well as the slip of the driven wheels. Referring to FIG. 24, a calculation result of the wheel speed variation D is plotted using white circles (◯) separately from black circles (xe2x97xaf) that indicates a wheel speed variation D when the effect of the tread-lift is not generated. Therefore, the accuracy of the calculation of the wheel speed variation D decreases, and therefore the warning pressure varies.
Further, if vehicle speed is high during the tire air pressure is decreasing, an accurate wheel speed variation D may not be obtained and therefore the tire air pressure decrease cannot be detected accurately.
For example, in the above-mentioned related device (JP-A-H10-100624), the wheel speed variation D under an ideal driven status is calculated based on the regression line. A difference (xc2x1Dxe2x80x2sh) between the wheel speed variation D under the ideal driven status and a predetermined reference value (Dxe2x80x2AVEstd) is defined as a determination value for determining the tire air pressure decrease. Then, the tire air pressure decrease is determined when the determination value exceeds a warning threshold. In this case, as shown in FIG. 25, the higher the vehicle speed is, the smaller the determination value is. That is, it is difficult to determine the tire air pressure decrease because the determination value is small. As a result, the tire air pressure decrease cannot be detected accurately.
It is therefore an object of the present invention to provide a tire air pressure detection device that is capable of obviating the above problems.
It is another object of the present invention to provide the tire air pressure detection device that is capable of increasing the accuracy of the rotational status value even if an effect of tread-lift is generated.
It is another object of the present invention to provide the tire air pressure detection device that is capable of obviating warning pressure non-uniformity.
A tire air pressure detection device of the present invention includes a warning anticipation timing calculating portion for anticipating a warning timing at which the tire air pressure decrease detecting portion will detect the tire air pressure decrease based on the rotational status value compensated for by a rotational status value compensating portion. A tire air pressure decrease detecting portion detects a decrease in tire air pressure when a clocked determination time reaches the warning anticipation timing anticipated by the warning anticipation timing calculating portion.
Accordingly, the driver is warned of the tire air pressure decrease when the clocked determination time reaches the warning anticipation timing regardless of the rotational status value compensated for by a rotational status value compensating portion. Therefore, even if the vehicle speed is high while the tire air pressure is decreasing and the rotational status value varies, the tire air pressure detection device can detect a decrease in tire air pressure when a determination reaches the warning anticipation timing.
A tire air pressure detection device of the present invention has a tread-lift compensation processing portion (3j) for determining whether a determination value exceeds a predetermined threshold (Cth) and for compensating for the determination value when the determination value exceeds the predetermined threshold. The tire air pressure decrease detecting portion detects a decrease in tire air pressure based on the determination value calculated by the tread-lift compensation processing portion.
The determination value is appropriately compensated for when the effect of tread-lift is large, and thus an appropriate determination value can be calculated. Therefore, the tire air pressure can be accurately detected.
A tire air pressure detection device of the present invention has a tread-lift compensation processing portion (3j) for removing an effect of tread-lift and adjusting a degree of the compensation of the determination value.
Accordingly, a degree of compensation of the determination value is adjusted with respect to an offset amount of the determination value. Therefore, the compensation degree increases when the effect of tread-lift is large, and decreases when the effect of tread-lift is small. As a result, an appropriate determination value can be calculated, and the tire air pressure can be accurately detected.
In a tire air pressure detection device of the present invention, the compensation methodology is executed after an initializing mode at which a post-compensation rotational status value is not yet calculated by a rotational status value calculating portion.
Accordingly, the determination value is not compensated for while a reference value is calculated when respective tire air pressures of respective vehicle wheels are identical. Therefore, initializing mode processing is appropriately executed.
A tire air pressure detection device of the present invention includes a first rotational status value compensation portion (3n) for compensating for the rotational status value calculated by the rotational status value calculating portion with respect to a vehicle speed as a parameter.
Accordingly, the rotational status value in which the effect of tread-lift is removed is calculated by compensating for rotational data within short intervals using vehicle speed as a parameter. As a result, it is possible to compensate for the slippage effect of the driven wheels based on the rotational status value, and to detect the tire air pressure decrease accurately.
In a tire air pressure detection device of the present invention, a regression line calculating portion (3d) is for classifying vehicle speed to several speed ranges, distinguishing the rotational status value and the slip status value in each related speed range of the several speed ranges, and calculating a regression line that is a linear function expressing a relationship between the rotational status value calculated by the rotational status value calculating portion and the slip status value calculated by the slip status value calculating portion at each of the several speed ranges. A rotational status value compensating portion (3f) is for compensating for the rotational status value calculated by the rotational status value calculating portion based on the regression line calculated at each speed range by the regression line calculating portion. A tire air pressure decrease detecting portion (3h) is for calculating a regression curve based on the rotational status value of each speed range compensated for by the rotational status value compensating portion, and for detecting a tire air pressure decrease based on a determination value (xcex94Dxe2x80x3AVE) that is defined based on the rotational status value at the each speed range compensated for by the rotational status value compensating portion.
According to the tire air pressure detection device of the present invention, the effect of tread-lift is removed after the effect of slip is removed. As a result, it is possible to detect the tire air pressure decrease accurately.